Optical recording medium having relationship between pit depths, wavelength and refractive index

ABSTRACT

A medium for optical recording is disclosed, from which recorded information is reproduced by a laser beam. The medium includes a disk board having a recording surface, and multiple pits in the recording surface. The pits are included in cells having equal size and varying pit-occupancy rates dependent on the recording information, the pit-occupancy rate being the ratio of area of the pit to area of the corresponding cell. The depths H of the pits, the wavelength λ of the laser beam, and the refractive index n of the board are related as, λ/6n&lt;H&lt;λ/4n.

TECHNICAL FIELD

The present invention generally relates to optical recording media andapparatuses, and more particularly to a medium for optical recording andan apparatus for optical information processing.

BACKGROUND ART

In recent years and continuing, an optical-recording medium such as aCompact Disk (CD) with a recording capacity of 0.65 Gigabytes and aDigital Video Disk (DVD) with a recording capacity of 4.7 Gigabytes isbecoming increasingly common as means for storing video information,voice information, or data on a computer. Moreover, at the present,there is an increasing demand for further improvement in recordingdensity and for increased capacity.

As means for increasing the recording density of such optical-recordingmedia, in an optical information processing apparatus which performswriting to or reading from the optical-recording media a set ofinformation, increasing the Numerical Apeture (NA) of an objective lensor shortening the wavelength of an optical source so as to reduce thediameter of a light spot formed on the optical-recording media by lightbeams condensed by the objective lens is effective.

Thus, for example, while the Numerical Apeture of the objective lens isset as 0.50 and the wavelength of the optical source as 780 nm for a“CD-type optical-recording medium”, the NA of the objective lens is setas 0.65 and the wavelength of the optical source as 660 nm for a“DVD-type optical-recording medium” with a greater recording density.Moreover, in order to respond to the demand for further improvement inthe recording density and increased capacity, further increasing of theNumerical Apeture of the objective lens above 0.65, or furthershortening of the wavelength of the optical source below 660 nm iscalled for. As for such large-capacity optical-recording media andoptical information processing apparatuses, there are systems proposedwhich establish the need for achieving the increased capacity using anoptical source in the blue-wavelength domain (refer to Non-PatentDocument 1, for example.)

However, there is a problem in which, as the NA of the objective lens isincreased, or the wavelength of the optical source is shortened, themargin for various changes in the optical-recording medium is reduced.For example, there is a problem in which a coma generated by a tilt inthe optical-recording medium becomes large. Generating of the coma leadsto a degrading of the beam spot formed on an information-recordingsurface in the optical-recording medium, preventing normal operations ofrecording and reproduction. The coma generated by the tilt in theoptical recording medium is generally given by the equation below:W ³¹=((n ²−1)/(2n ³))×(d×NA ³×θ/λ)

Herein, n represents the refractive index of a transparent board in theoptical recording medium, d the thickness of the transparent base board,NA the Numerical Apeture (NA) of the objective lens, λ the wavelength ofthe optical source, and θ the amount of the tilt. It may be understoodfrom the equation that the shorter the wavelength and the higher the NA,the bigger the coma. Similarly, as a spherical aberration generated by adifference in the thickness of the board in the optical recording mediumis proportional to the 4^(th) power of NA, the Numerical Apeture, and tothe (−1)^(th) power of λ, the wavelength, the shorter the wavelength andthe higher the NA, the bigger the spherical aberration.

Thus, as another proposal, a multi-level recording and reproductionmethod of controlling the signal levels in pits formed on the opticalrecording medium is being proposed. In other words, in a related-artoptical recording medium, reading of a signal is performed by using achange in the amount of light reflected dependent upon the presence orabsence of pits when scanning over the optical recording medium with alaser light for reading. The multi-level recording and reproductionmethod, as described in Patent Document 1, which performs the reading ofthe signal by using the change in the amount of light reflecteddependent upon a combination of pit depth and pit width; and themulti-level recording and reproduction method, as described in PatentDocument 2, which performs the reading by using the change in the pitdepth and the pit width, and an offset in the pit position, are alsobeing proposed.

Furthermore, in Patent Document 3, concerning a method of recordingmulti-level information using a recording mark occupancy rate method, ina case of a concavo-convex-shaped phase pit, a setting of anoptical-channel depth of the phase pit as λ/4 so as to make a signalgain of a Radio-frequency (Rf) signal maximal is described.

Patent Document 1

JP58-215735A

Patent Document 2

JP07-121881A

Patent Document 3

JP2002-157734A

Patent Document 4

JP04-030094A

Non-Patent Document 1

Hiroshi Ogawa, “Next Generation Optical Disc”, Proceedings of the ISOM,pp. 6-7, 2001

However, as described in the Patent Documents 1 and 2, the efficiency ofproducing the optical recording medium in which multi-level data arerecorded using the pit depth and the pit width becomes very low.

FIG. 14 and FIG. 15 illustrate a manufacturing process of areproduction-only optical recording medium in which a binary signalcomprising a low-level and a high-level is recorded in related-artCD-type and DVD-type pits. The known manufacturing process comprises thesteps of laser cutting (S1), developing (S2), stamper production (S3),and replication (S4). In other words, first, as illustrated in FIG. 15A,using an optical beam 100, a portion to be the pit 1 on a resist surface102 of a glass board 101 is exposed to the depth where the optical beam100 falls upon the glass board 101, the developing is performed asillustrated in FIG. 15B, a stamper master 103 is produced based on thedeveloped originally-recorded board as illustrated in FIG. 15C. Then,using the stamper master 103, according to a known replication process,reproduction-only optical recording media 104 are manufactured in largequantities. The number 105 represents a groove, while 106 represents aland.

In the related-art manufacturing process, there is a need to controlexposure of the glass board 101 so as to comprise pits having aplurality of depths. However, in such a case, the bottom losessmoothness (surface curvature or surface roughness is produced).Moreover, it is empirically understood that there are problems such thata change in the depth is very sensitive to a change in the exposure soas to make the controlling difficult. While depth modulation istheoretically enabled by changing the exposure, due to such reasons, ina related-art optical recording medium for CD reproduction or arelated-art optical recording medium for DVD reproduction, the exposingof the glass board so as to record the signal which corresponds to apit-length modulation is not performed.

Moreover, as another problem to be solved by the present invention,there is a method of generating a tracking-error signal. In other words,the method of generating the tracking-error signal which has beenadopted by the optical recording medium for CD reproduction and theoptical recording medium for DVD reproduction is not suitable for suchshortened wavelength, increased NA, increased density or speed usingsuch methods as the multi-level recording method. Also it is a method bywhich it is difficult to achieve compatibility with a recording-typeoptical recording medium. In other words, a Differential Push-Pullmethod (referred to as the DPP method below) is applied to the opticalrecording medium for CD reproduction, while a DifferentialPhase-Detection method (referred to as the DPD method below) is appliedto the optical recording medium for DVD reproduction. Below, therespective methods are described.

The Differential Push-Pull method (or the DPP method) is described.Generating the tracking-error signal by the DPP method comprises using agrating on a main track and its neighboring track, a main beam M1 spot,and first and second sub-beam S1 and S2 spots.

Referring to FIG. 16 which illustrates a related-art optical pickup (anoptical element of an optical information processing apparatus), a beamoutput from a semiconductor laser 110 is collimated at a collimator lens111 so as to be diffracted at a grating 112 for branching intozeroth-order and ± first-order diffracted lights. Then, such branchedlights, at the object lens 113, are condensed so as to illuminate anoptical recording medium 114. Then, as illustrated in FIG. 17, the mainbeam M1 spot comprising the zeroth-order diffracted light is formed overa main track T, and the first and the second subbeam S1 and S2 spotscomprising the ± first-order diffracted lights, respectively, precedeand follow relative to the main beam M1 spot so as to be formed in aradial direction of the optical recording medium 114 with a separationdistance from each other of ±½ track pitch.

The main beam M1 and the first and the second subbeams S1 and S2reflected at the optical recording medium 114 so as to be passed throughthe object lens 113 are deflected from the illuminated light beam at abeam splitter 115 so as to be received at receiving optics 117 via adetector lens 116. The receiving optics 117, as illustrated in FIG. 17,comprises a first optical detector 118 a which receives the main beamM1, and a second optical detector 118 b and a third optical detector and118 c which respectively receive the first and the second subbeams S1and S2. Each of the first through third optical detectors 118 a through118 c independently converts the beam photoelectrically, using twoplates partitioned in the radial direction of the optical recordingmedium 114.

The signals detected at the first through third optical detectors 118 a,118 b, and 118 c are respectively input to first through thirddifferential amplifiers 119, 120, and 121 so as to be output as firstthrough third push-pull signals. A first amplifier 122 amplifies to apredetermined gain G1 the third push-pull signal, while a secondamplifier 123 amplifies to a predetermined gain G2 a signal summing asignal output from the first amplifier 122 and the second push-pullsignal. A fourth differential amplifier 124 takes the difference betweena signal input from the second amplifier 123, and the first push-pullsignal, based on the main beam M1, input from the first differentialamplifier 119. Then, the gains G1 and G2 of the first and the secondamplifiers 122 and 123 are determined by taking into account intensitiesof the main beam M1 and the first and the second beams S1 and S2.

While there is no particular problem in adopting the DPP method for thereproduction-type optical recording medium, in a case of adopting themethod for operating the recording-type optical recording medium usingthe optical pickup apparatus having such configuration, problems asdescribed below arise. In other words, as for the beam output from theoptical source 110 diffracted at the grating 112 into three beams M1,S1, and S2 so as to be used, there are problems as follows:

(1) The optical efficiency of the main beam M1 decreases so as to makeits use for recording difficult. Moreover, an application requiringincreased speed becomes difficult.

(2) Moreover, at the time of the recording, a signal recorded on aneighboring track may be erased by the first and the second subbeams S1and S2.

Next, the Differential Phase-Detection method (the DPD method) isdescribed. In the DVD reproduction-type optical recording medium, amethod called the Differential Phase-Detection method, or DPD method isadopted as a method for obtaining the tracking-error signal. This methoduses a change, at the time a beam spot illuminating an optical recordingmedium passes over the pit, of an image (a diffraction pattern) of thepit on the receiving optics due to an offset, from the center of thepit, of the beam spot, the receiving optics being arranged so as tocomprise areas, partitioned in the longitudinal direction of the track,of the image of the pit. The way of changing the output signal level, inresponse to the amount of light received at the respective areas,differs depending upon the direction and magnitude of the offset, fromthe center of the pit, of the beam spot. The tracking-error signalindicating the direction and the magnitude of the offset of the beamspot is obtained by binarizing the output of the receiving optics topredetermined levels so as to compare the phases of the binarizedsignal, and to examine which changed earlier and the time difference(phase difference) of the changing in the level.

FIG. 18A through FIG. 20B illustrate variations of distribution patterns(far-field images) of intensities of the amount of light reflected,received at the optical receiving areas a, b, c, and d of thequad-partitioned receiving optical sensors 133 at the time a beam spot121 passes over information pits 132, a set of FIG. 18A, FIG. 19A, andFIG. 20A and a set of FIG. 18B, FIG. 19B, and FIG. 20B respectivelyrepresenting the positional relationships between the beam spot 121 andthe information pits 132, and the far-field images of the amount oflight reflected when the beam spot 121 passes over the information pits132. At the time the beam spot 121 is over an information track of therecording medium, the far-field image is in even brightness. In a caseof the beam spot 121 passing over the center of the information pits132, as illustrated in FIG. 19A and FIG. 19B, the far-field imagechanges while keeping bilateral symmetry. Moreover, as illustrated inFIG. 18A, FIG. 18B, FIG. 20A, and FIG. 20B, in a case of the beam spot121 passing over with an offset from the center of the information pits132, the bilateral symmetry in the far-field image is lost, causing thetime difference (the phase difference) in the way of the change, and ina case of the beam spot 121 passing over the right-hand side of thecenter of the information pits 132, as illustrated in FIG. 18B, changingso as to rotate clockwise, while conversely in a case of the beam spot121 passing over the left-hand side of the center of the informationpits 132, as illustrated in FIG. 20B, changing so as to rotatecounterclockwise. Such changes in the pattern become clearer with thebeam spot 121 comprising the offset from the center of the informationpits 132. Hereby, performing a conversion of the amount of light to anelectrical signal so as to detect the time difference makes it possibleto obtain the tracking-error signal as illustrated in FIG. 21.

By subtracting the tracking-error signal, as in FIG. 18A through FIG.20B from the sum of the output signal levels corresponding to theamounts of reflected light received at the optical receiving areas a, b,c, and d, positioned diagonally from one another, of thequad-partitioned optical receiving sensors 133, the difference among thesignal levels as described above becomes zero, representing a statereferred to as an on-track state in which the beam spot 121 is rightabove the track. On the other hand, as the beam spot 121 is separatedfrom the information pits, in response to the amount of light, thesymmetry of the distribution of the intensities in the far-field imageis lost so that the tracking-error signal is generated.

However, in a large-capacity optical recording medium where the beamdiameter is greater than the pit diameter, in a case such that multipleedges exist in a beam (referring to FIG. 6A), the DPD method can not beused as multiple diffraction patterns mix with one another.

Moreover, while the DPD method is used for the DVD reproduction-typemedium, for a DVD recording-type optical recording medium, the DPPmethod and a push-pull method are used so that there is a need toswitch, between the recording-type and the reproduction-type, the methodof generating the tracking-error signal.

Furthermore, in a case of using the recording mark occupancy rate methodas described in Patent Document 3, as the optical-channel depth of thephase pit is set to be λ/4, obtaining the push-pull signal is almostimpossible so as not to be able to apply the push-pull method indetecting the tracking-error signal.

DISCLOSURE OF THE INVENTION

Accordingly, it is a general object of the present invention to providean optical recording medium and an optical information processingapparatus that substantially obviate one or more problems caused by thelimitations and disadvantages of the related art.

It is a more particular object of the present invention to provide areproduction-type optical recording medium in which multi-levelinformation is recorded, enabling ease in production, as well as toensure, for the reproduction-type optical recording medium, ease incompatibility of the tracking-error signal with a recording-type opticalrecording medium.

It is another more particular object of the present invention to enablethe application, to a reproduction-type optical recording medium with anenlarged capacity (or with smaller-sized pits), of an easy method forgenerating a tracking-error signal.

It is another particular object of the present invention to enable theapplication of a method for generating a track-error signal withouthaving such problems in a recording-type optical recording medium as areduction of the amount of light or cross-erasing an adjacent track.

According to the invention, in a medium for optical recording, fromwhich recorded information is reproduced by a laser beam, including adisk board having a recording surface, and multiple pits in therecording surface, the pits are included in corresponding cells eachhaving equal size and varying pit-occupancy rates dependent on therecording information, the pit-occupancy rate being the ratio of theareas of the pits to the area of the corresponding cell, wherein depthsH of the pits, a wavelength λ of the laser beam, and a refractive indexn of the board are related as, λ/6n<H<λ/4n.

A medium for optical recording in an embodiment of the invention enablesthe maintenance of a stable developing process which performs anexposure on the glass board so as to provide a reproduction-type opticalrecording medium in which multi-level information is recorded, which iseasy to produce by partitioning an area comprising the pits intomultiple cells having areas equal to one another, so as to include percell one area-modulated pit in which reproduced signals having multiplelevels dependent upon the pit occupancy rate relative to the respectivecells are generated, as the multi-level information is recorded not bydepth modulation, but rather by area modulation. Then, detection of thetracking-error signal also in a reproduction-type optical recordingmedium comprising pits using a push-pull method generally used in arecording-type medium is enabled so as to achieve compatibility withgenerating a tracking-error signal in a recording-type optical recordingmedium by setting pit depth within a range from λ/6n where the push-pullamplitude is maximal to λ/4n where amplitude differences among therespective pit sizes of the reproduced signals are maximal, orsignal-to-noise ratio of the reproduced signals is maximal, especiallyas the intermediate value of the range.

According to another aspect of the invention, an apparatus for opticalinformation processing includes an illumination optical system, areceiving optical system which optically receives light reflected fromthe medium for optical recording, and a signal-processing section whichperforms, based on signals detected which are optically received at thereceiving optical system, a processing of the signals, wherein thereceiving optical system comprises at least a pair of first receivingoptical sections arranged symmetrically in a radial direction of themedium for optical recording within an area in a far-field in which azeroth-order light reflected and ± first-order diffracted lightsreflected from the pits overlap, and the signal-processing sectiondetects a tracking-error signal, based on differences among the at leastthe pair of first receiving sections of the signals detected at thefirst receiving optical sections, using a push-pull method.

An apparatus for optical information processing in an embodiment of theinvention enables the detection of a tracking-error signal also in anoptical recording medium including the pits as described above using apush-pull method generally used in a recording-type optical recordingmedium so as to achieve compatibility with generating the tracking-errorsignal in a recording-type optical recording medium as well as resolvinga problem of the DPD method, as observing the edges between the pits inthe motion direction of the beam spot is not needed, and such problemsas optical efficiency and cross-erasing, because of one-beam tracking asdescribed above.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features, and advantages of the present invention willbecome more apparent from the following detailed descriptions when readin conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of an optical-pickup apparatus accordingto a first embodiment of the present invention;

FIG. 2 is a graph which illustrates relationships among pit depths, pitdiameters, and push-pull signal amplitudes;

FIG. 3A through FIG. 3F are characteristic diagrams of the push-pullsignals for the different pit depths;

FIG. 4 is a graph which illustrates relationships among the pit depths,the pit diameters, and reproduced signals (Rf signals);

FIG. 5A through FIG. 5F are graphs of the Rf signals for the differentpit depths;

FIG. 6A is a schematic explanatory drawing which illustrates a beam spotin relative motion over pits;

FIG. 6B is a schematic explanatory drawing which illustrates guiding abeam of light reflected to an optical detector;

FIG. 7 is a schematic explanatory drawing which illustrates theappearing of a push-pull signal and a Rf signal.

FIG. 8 is a graph which illustrates relationships among the pitdiameters (pit occupancy rates), the pit diameters, and the Rf signals;

FIG. 9 is an explanatory drawing which illustrates the corresponding Rfsignal levels for the respective pit sizes, assuming that the number ofpit patterns is 8;

FIG. 10 is a block diagram which illustrates receiving optics with apartitioned structure and a signal processor, according to a thirdembodiment of the present invention;

FIG. 11A is a graph of push-pull signals indicating a presence or anabsence of an effect of an object-lens shift, according to methods inembodiments of the present invention;

FIG. 11B is a graph of push-pull signals indicating a presence or anabsence of an effect of an object lens shift, according to a related-artmethod;

FIG. 12 is a schematic block diagram which illustrates an optical pickupapparatus according to a fourth embodiment of the present invention;

FIG. 13 is a schematic diagram which illustrates a combination of ahologram and receiving optics;

FIG. 14 is a flowchart which illustrates a manufacturing process of areproduction-type optical recording medium, according to therelated-art; FIGS. 15A-D are schematic cross-sectional diagrams whichillustrate specifics of the manufacturing process as illustrated in FIG.14;

FIG. 16 is a schematic diagram which illustrates an optical pickupapparatus so as to describe a related-art DPP method and the secondembodiment of the present invention;

FIG. 17 is a schematic diagram which illustrates a detecting opticalsystem in the optical pickup apparatus as illustrated in FIG. 16;

FIG. 18A and FIG. 18B are explanatory drawings which illustrate a beamspot and optical receiving states so as to describe a related-art DPDmethod;

FIG. 19A and FIG. 19B are explanatory drawings which further illustratethe beam spot and the optical receiving states so as to describe therelated-art DPD method;

FIG. 20A and FIG. 20B are additional explanatory drawings which furtherillustrate the beam spot and the optical receiving states so as todescribe the related-art DPD method; and

FIG. 21 is an explanatory drawing which illustrates obtaining thetracking-error signal.

BEST MODE FOR CARRYING OUT THE INVENTION

Descriptions are given next, with reference to the accompanyingdrawings, of embodiments of the present invention.

The present invention is not limited to the specifically disclosedembodiments, but variations and modifications may be made withoutdeparting from the scope of the present invention.

A first embodiment of the present invention is described based on FIG. 1through FIG. 9. The present embodiment basically relates to areproduction-type optical recording medium in which a reproduced signalis generated from the amount of light reflected of a laser lightilluminating pits. The structure in an area comprising pits ispartitioned into multiple cells having equal areas, and anarea-modulated pit, which generates reproduced signals having multiplelevels dependent upon pit occupancy rates for the respective cells, iscomprised per cell.

In order to prove the viability of such a reproduction-type opticalrecording medium, a validation was performed. The optical recordingmedium 4 has various pit depths and pit diameters at a track pitch of0.43 μm. As schematically illustrated in FIG. 1, an optical pickupapparatus comprising an illumination optical system 3 has an opticalsource 1 which generates a laser beam with a wavelength of 400 nm and anobject lens 2 having a NA of 0.65. Herein, 5 represents a collimatedlens which collimates the laser beams, 6 a beam splitter which passesilluminating light and deflects returned light, and 7 a detecting lenswhich collects the deflected light reflected to an optical detector 8,comprising together a receiving optical system 9. Furthermore, as amethod of detecting a tracking-error signal, a push-pull methodgenerally adopted to a recording-type optical recording medium is used.

A result of the validation is described. First, FIG. 2 illustratesrelationships among pit depths, pit diameters, and push-pull signalamplitudes, the horizontal axis representing the pit depths and thevertical axis representing the push-pull signal amplitudes, plottingmultiple curves for different pit diameters. The push-pull signalamplitudes as illustrated in FIG. 2 are results obtained from push-pullsignal characteristics as illustrated in FIG. 3A through 3F. FIG. 3Athrough 3F correspond to the push-pull signal characteristics for therespective pit depths of 150 Å, 300 Å, 400 Å, 510 Å, 660 Å, and 800 Å,and, furthermore, multiple curves for the different pit diameters areplotted in FIG. 3A through 3F. Every one of FIG. 3A through FIG. 3Fillustrates cases of pit diameters of 0.230 μm, 0.207 μm, 0.184 μm,0.161 μm, 0.138 μm, 0.115 μm, 0.092 μm, and 0.069 μm in descending orderof the push-pull signal amplitude.

Assuming that the maximum level of the curves of FIG. 3A through 3F isPP_(max), and the minimum level PP_(min), the Push-Pull amplitude (PP)may be represented as PP=(PP_(max)−PP_(min)). The push-pull signalamplitude reaches the maximum with the pit depth of 0.04 μm (orapproximately λ/6n) and the pit diameter of 0.069 μm, and the minimum atthe pit depth of 0.064 μm (or approximately λ/4n) and the pit diameterof 0.023 μm.

Moreover, FIG. 4 illustrates relationships among the pit depths, the pitdiameters, and reproduced signals (Rf signals), the horizontal axisrepresenting the pit depths and the vertical the Rf signals at anon-track timing, plotting multiple curves corresponding to the differentpit diameters. Furthermore, the relationships between the Rf signals asillustrated in FIG. 4 and track positions are as illustrated in FIG. 5Athrough FIG. 5F. FIG. 5A through FIG. 5F correspond to thecharacteristics of the Rf signals for the different pit depths and trackpositions, and plot multiple curves for the different pit diameters.FIG. 5A through FIG. 5F, as in FIG. 3A through FIG. 3F, illustrate therespective cases in which the pit diameters are 0.230 μm, 0.207 μm,0.184 μm, 0.161 μm, 0.138 μm, 0.115 μm, 0.092 μm, and 0.069 μm in thedescending order of Rf signal amplitude.

The levels at the center of the track of the respective curves in FIG.5A through FIG. 5F correspond to FIG. 4. The Rf signal reaches a minimumwith the pit depth of 0.066 μm (approximately λ/4n), and the pitdiameter of 0.230 μm.

Next, the push-pull signal and the Rf signal are described, referring toFIG. 6 and FIG. 7. FIG. 6A illustrates a beam spot 12 in relative motionover pits 11, while FIG. 6B illustrates the beam spot 12 of reflectedlight, guided to an optical detector 8 comprising receiving opticalareas A and B symmetrically dual-partitioned in a radial direction. Inother words, the optical detector 8 comprises the partitioned receivingoptical areas A and B as at least a pair of a first receiving opticalsection arranged in the radial direction of the optical recording medium4 within an area in far-field in which a zeroth-order reflected lightand ± first-order diffracted lights overlap. Moreover, according to thepresent embodiment, the pits 11 are especially set to have circular(geometrically circular) patterns having different radii depending uponarea modulation, so that one of the pits is comprised, perequally-partitioned area referred to as a cell 13, in the centralposition of the area.

The push-pull signal, TE1, and the reproduced signal, Rf, which aretracking-error signals, may be obtained by computations below usingdetected signals a and b at the partitioned receiving optical areas Aand B:TE1=(a−b)/(a+b)Rf=(a+b)/(a+b)

Herein, an appearing of the push-pull signal and the Rf signal asillustrated in FIG. 3 and FIG. 5 is described, referring to FIG. 7. TheRf signal is a summed signal of the amount of light returned to thepartitioned receiving optical areas A and B when the beam spot 12illuminates the optical recording medium 4. When the beam spot 12 ispositioned over the pits 11, as light is affected by diffraction at thepits 11 so that the amount of light returned to the partitionedreceiving optical areas A and B (or, the amount of light reflected)decreases, the Rf signal level decreases. On the other hand, when thebeam spot 12 illuminates the optical recording medium 4, the push-pullsignal indicates an unbalance of the amount of light reflected in aradial direction of the pits 11. As the beam spot 12 approaches edges ofthe pits 11, the direction of the diffraction of light biases to alongitudinal direction of the pits 11, the biased direction differingdepending on whether the edges precede or follow the pits 11 so that asthe difference between outputs a and b of the partitioned receivingoptical areas A and B is computed, pulse-shaped signals having differentpolarities for the preceding and the following edges relative to thepits 11 are obtained.

Referring to FIG. 5, it may be understood that, even for a predeterminedpit depth, multiple Rf signal levels may be obtained by changing the pitdiameter. In other words, changing the pit diameter enables recordingand reproduction of multi-level data. Moreover, referring to FIG. 2 andFIG. 3, it may be understood that, even in an optical recording mediumhaving a smaller pit diameter relative to the beam spot diameter, undera predetermined pit condition, detection of a push-pull signal isenabled. As described above, considering the compatibility with arecording-type optical recording medium comprising a continuouschannel-shaped structure, the push-pull method is desired as a method ofdetecting a tracking-error signal.

As described above, as for the depths of the pits 11, values within therange from λ/6n, where the maximum for the push-pull signal amplitude isreached to λ/4n where the maximum in the differences among the pit sizesin the Rf signal amplitudes is reached, and especially the intermediatevalues (at around λ/5n), may be selected. The latter maximum in thedifferences in the Rf signal amplitudes indicates that thesignal-to-noise ratio of the Rf signal reaches the maximum.

Moreover, the method of reproducing such multi-level recorded data isdescribed. The relationships between the pit diameters (the pitoccupancy rates) and the Rf signals are as illustrated in FIG. 8. Thedifference from FIG. 4 is that the horizontal axis is represented by thepit diameter and, especially herein, the pit depth is set to be λ/6n.The Rf signal level changes depending upon the occupancy rate of thepits 11 in one cell 13. When pits 11 do not exist, the Rf signal reachesthe maximum level, while it reaches the minimum level when the occupancyrate of the pits 11 reaches a maximum. Using such relationships, in FIG.9, the Rf signal levels corresponding to the respective pit sizes areillustrated in a case that the number of pit patterns (the number ofmultiple levels is 8 (levels 0 through 7). Adopted patterns of the pits11 are illustrated in an upper portion of FIG. 9.

In other words, in a case where the reproduced signals having multiplelevels dependent upon the pit occupancy rates for the cells 13 aregenerated, the pits 11 comprise (N−1), or 7 different pit diametersbased on the area modulation, the seven different pit diameters beingset so as to approximately equally divide into eight parts thedifference in the amount of light reflected from the cell 13 in a caseof the pits 11 comprising a maximum pit diameter (in which Rf signallevel is minimal) and the amount of light reflected from the cell 13 ina case in which pits 11 do not exist (in which Rf signal level ismaximal.) Therefore, according to the present embodiment, thereproduction-type optical recording medium is produced by partitioningthe area comprising the pits 11 into the cells 13 having equal areas soas to comprise per cell 13 one of the pits 11, which is area-modulated,in which reproduced signals having multiple levels dependent upon thepit occupancy rates for the cells 13 are generated. Multi-levelinformation is recorded not by depth modulation, but rather by areamodulation so that, in such a manufacturing process, a stable developingprocess of exposing the glass board and a provision of areproduction-type optical recording medium, in which multi-levelinformation is recorded, with an ease in production is obtained.Moreover, a setting of pit depths within the range from λ/6n where thepush-pull signal amplitude is maximal to λ/4n where the amplitudedifferences among the pit sizes of the reproduced signals are maximal,or in other words, the signal-to-noise ratio of the reproduced signal ismaximal, especially the intermediate values in the range, enablesdetecting the tracking-error signal based on the push-pull methodgenerally used in a recording-type optical recording medium. In otherwords, in a case where reproduced information is recorded per cell usingthe pit occupancy rates, the result is that an arranging of the pitslined up continuously is observed equivalently by the beam spot 12 as anarranging of channels comprised continuously, enabling achievingcompatibility with the generating of the tracking-error signal of therecording-type optical recording medium. In such case, as there is noneed to observe the edges between the pits in the motion direction ofthe beam spot 12, the problem of the DPD method as well as such problemsas the optical efficiency and cross-erasing are resolved.

A second embodiment according to the present invention is describedusing FIG. 16 and FIG. 17.

The present embodiment relates to generating a tracking-error signal forthe reproduction-type optical recording medium 4 as described above, anda configuration of a receiving optical system and signal-processingsystem different from the first embodiment. In other words, while, in acase of using a simple push-pull method as described above, there is aproblem in which an offset is generated in a tracking-error signal dueto an imbalance in the distribution of light on a surface of the opticalreceiving section when an optical axis of an object lens 2 shifts (ashift) or tilts (a tilt) from an optical axis of a fixed optical system.The present embodiment enables resolving such a problem by using agenerally-known differential push-pull method.

The differential push-pull method (the DPP method) generates atracking-error signal by comprising, in an optical recording mediumusing a grating, on a main track and on a neighboring track of anoptical recording medium, a main beam M1 spot and first and the secondsub-beam S1 and S2 spots.

Referring to FIG. 16 which illustrates an optical pickup (an opticalelement of an optical information processing apparatus) adopting agrating, an output beam from a semiconductor laser 110 is collimated ata collimating lens 111 so as to be diffracted at a grating 112 to bebranched into a zeroth diffracted light and ± first diffracted lights.Then, such branched lights are, at an object lens 113, as illustrated inFIG. 17, condensed to illuminate an optical recording medium 114. Then,the main beam M1 spot which is the zeroth diffracted light is formedover a main track T of the optical recording medium 114, while the firstsub-beam S1 spot and second sub-beam S2 spot, which are ± first-orderdiffracted lights, respectively, precedes and follows, respectively, themain beam M1 spot, formed in a radial direction of the optical recordingmedium 114 with a distance of a ½ track pitch apart.

The main beam M1 and the first and second sub-beams S1 and S2 reflectedat the optical recording medium 114 so as to pass through the objectlens 113 are deflected from illuminating light at a beam splitter 115 soas to be received at receiving optics 117 via a detecting lens 116. Thereceiving optics 117, as illustrated in FIG. 17, comprises a firstoptical detector 118 a which optically receives the main beam M1 andsecond and third optical detectors 118 b and 118 c which respectivelyoptically receive the first and second sub-beams S and S2. Such firstthrough third optical detectors 118 a through 118 c comprise plates,dual-partitioned in the radial direction of the optical recording medium114, the plates independently performing photo-electrically converting.

The signals detected at the first through third optical detectors 118 a,118 b, and 118 c are input to the first through third differentialamplifiers 119, 120, and 121, respectively, so as to be output as thefirst through third push-pull signals. The first amplifier 122 amplifiesthe third push-pull signal to a predetermined gain G1, while the secondamplifier 123 amplifies a signal summing a signal output from the firstamplifier 122 and the second push-pull signal to a predetermined gainG2. The fourth differential amplifier 124 takes the difference between asignal input from the second amplifier 123 and the first push-pullsignal, based on the main beam M1, input from the first differentialamplifier 119 so as to be output as a tracking-error signal. Then, thegains G1 and G2 of the first and second amplifiers 122 and 123 aredetermined, taking into account the intensities of the main beam M1 andthe sub-beams S1 and S2. Thus, even in a case where the object lens 2 isshifted in the radial direction of the optical recording medium 4, apush-pull signal (a tracking-error signal) which includes almost nopush-pull offsets is obtained.

A third embodiment according to the present invention is described,referring to FIG. 10 and FIG. 11. The portions which are the same as theportions illustrated in the first embodiment are illustrated using thesame numerals, and their explanations are abbreviated (and treated thesame in the embodiments below).

The present embodiment relates to a generating a tracking-error signalfor the reproduction-type optical recording medium 4 as described above,the configurations of the receiving optical system and the signalprocessing system being different from the first and the secondembodiments. In other words, in a case of using a simple push-pullmethod as in the first embodiment, there is a problem of generating anoffset to a tracking-error signal due to an imbalance in thedistribution of light on the surface of an optical receiving sectionwhen an optical axis of an object lens 2 shifts (a shift) from anoptical axis of a fixed optical system or tilts (a tilt). On the otherhand, in a case of using the DPP method as in the second embodiment, asa beam output from the optical source 110 is used by being diffractedand branching into three beams M1, S1, and S2 at a grating 112, (1)optical efficiency of the main beam M1 decreases so that use inrecording or applying to an increased speed becomes difficult, (2) thereis a problem in that, at the time of the recording, the recorded signalsat neighboring tracks are erased by the first and second sub-beams S1and S2. On the other hand, according to the present embodiment, suchproblems are solved by using a compensation-type push-pull method asdescribed, for example, in Patent Document 4.

Therefore receiving optical system 9 comprises, as an optical detector,receiving optics 21 having a structure quad-partitioned in a radialdirection, as illustrated in FIG. 10. The receiving optics 21 comprisesfour receiving optical areas A, B, C, and D, having as a first receivingoptical section a pair of the receiving optical areas A and B arrangedsymmetrically relative to the radial direction of an optical recordingmedium 4 within an area in a far-field in which a zeroth-order light and± first-order diffracted lights reflected from the pits 11 overlap withone another and as a second receiving optical section a pair of thereceiving optical areas C and D arranged symmetrically relative to theradial direction of the optical recording medium 4 within the area inthe far-field with only the zeroth-order light reflected from the pit11. Assuming signals detected at the receiving optical areas A through Das a through d, a signal processing section 22 performs signalprocessing based on such detected signals a through d so as to detectsuch signals as tracking-error signals. The signal processing section 22comprises a differential amplifier 23 which obtains a difference signalfrom the signals detected a and b at the pair of the receiving opticalareas A and B as the first receiving optical section, a differentialamplifier 24 which obtains a difference signal of the signals detected cand d at the pair of the receiving optical areas C and D as the secondreceiving optical section, an adder 25 which adds the results ofcomputing at the differential amplifiers 23 and 24, and a gain-adjustingunit 26, arranged between the differential amplifier 24 and the adder25, variably controlling a gain K as means for correcting so as toamplify or decrease the output of the differential amplifier 24.

Hereby, a tracking-error signal TE2 in the case of the presentembodiment is obtained according to an equation below:TE 2=[(a−b)+K(c−d)]/[(a+b)+K(c+d)]Hereby, adjusting the gain K of the gain adjusting unit 26 so as tominimize a push-pull offset included in such compensation-type push-pullsignal TE2 as described above enables the obtaining of a push-pullsignal (a tracking-error signal) TE2 which contains almost no push-pulloffsets.

The point as described above is explained in detail, referring to FIG.11. First, FIG. 11 illustrate signals detected, using an optical source1 which outputs a beam with a wavelength λ of 400 nm and an object lenswith a NA of 0.65, emitting a beam illuminating pits 11 having pitdepths of λ/6n and pit diameters of 0.23 μm in an optical recordingmedium 4 having a track pitch of 0.43 μm so as to optically receive thereflected light. Herein, while FIG. 11B illustrates a tracking-errorsignal TE1 using a generally-used push-pull method, a larger push-pulloffset is generated by a radial shift in the object lens 2 asillustrated by a solid line and, as the amount of shift in the radialdirection of the object lens 2 becomes larger (towards the right-handside of the x-axis), such amount of the push-pull offset increases.

On the other hand, FIG. 11A, illustrates that applying thecompensation-type push-pull method enables detecting a tracking-errorsignal TE2 with almost no push-pull offsets even in a case of the objectlens 2 being shifted in the radial direction. In other words, even in acase of the configuration as illustrated in FIG. 10, (a−b) and (c−d),comprising the push-pull signal as in the tracking-error signal TE1,include considerable push-pull offsets so that, with an increase in theamount of shift in the object lens 2, the amounts of the push-pulloffsets increase. However, multiplying by the gain K at thegain-adjusting unit 26 of the push-pull signal (c−d) so as to add to thepush-pull signal (a−b) at the adder 25 enables, as illustrated in FIG.12, detecting a tracking-error signal TE2 which contains almost nopush-pull offsets relative to the shift in the radial direction of theobject lens 2.

Incidentally, the method of generating the compensation-typetracking-error signal TE2 according to the present invention may beapplied not only to the reproduction-type optical recording medium 4according to the present invention as described above, but also to therecording-type optical recording medium (comprising compatibility),wherein guiding channels (grooves) run continuously on aninformation-recording surface of a recording-type optical recordingmedium, the surface being coated with such materials as a phase-changingmaterial so as to make different reflectances. Thus, in a case ofincluding as targets a recording-type optical recording medium with thecontinuously-running grooves, in addition to the reproduction-typeoptical recording medium, switching to adjust a predetermined gain K,depending upon whether it is a reproduction-type optical recordingmedium or a recording-type optical recording medium, enables generatinga tracking-error signal suitable for the characteristics of therespective optical recording media.

Moreover, in cases of applying the pit structure comprising themulti-level information not only throughout the surface but alsopartially (a ROM area comprising the pits), and, furthermore, in a caseof including as targets a hybrid-type optical recording medium alsohaving a RAM area with continuously-running grooves, switching to adjusta predetermined gain K depending upon whether it is the ROM area or theRAM area enables generating a tracking-error signal suitable for thecharacteristics of the respective areas.

A fourth embodiment according to the present invention is describedusing FIG. 12 and FIG. 13. While the present embodiment basically is thesame as the third embodiment, applying a compensation-type push-pullmethod, a receiving optical system 9 uses, in lieu of receiving optics21, a configuration combining a hologram (a diffracting device) 31 withmultiple receiving optics 32A through 32D.

First, the hologram 31, as in the case of the receiving optics 21,comprises four diffracting areas A, B, C, and D quad-partitioned in aradial direction, comprising a pair of the diffracting areas A and Bcomprising hologram patterns, arranged symmetrically relative to theradial direction of an optical recording medium 4. First-order inputlight is diffracted at different angles, within an area in a far-fieldin which a zeroth-order light reflected and the ± first-orderdiffracting lights reflected from the pits 11 overlap. A pair ofdiffracting areas C and D comprise hologram patterns, arrangedsymmetrically relative to the radial direction of the optical recordingmedium 4, so as to first-order diffract input light at different angles,within the area in the far-field with only the zeroth-order lightreflected from the pits 11. Then, a combination of receiving optics 32Aand 32B arranged at the positions of optically receiving the deflectedlight diffracted by the diffracting areas A and B comprises a firstreceiving optical section while a combination of receiving optics 32Cand 32D arranged at the positions of receiving the deflected lightdiffracted by the diffracting areas C and D comprises a second receivingoptical section.

In the case of the present embodiment, the hologram 31 is comprisedwithin the receiving optical system 9 and the signals detected a throughd at the receiving optics 32A through 32D are the same as in the case ofthe signals detected a through d by the receiving optics 21, so detailsare abbreviated. A configuration, which is the same as the signalprocessing section 22 as illustrated in FIG. 10, enables generating atracking-error signal TE2 so as to minimize the push-pull offset.

Therefore, in the case of the present embodiment, as in the case of thethird embodiment, generating a tracking-error signal TE2 not affected bya push-pull offset is enabled. Arranging a hologram 31 having apartitioned structure at former stages of the receiving optics 32Athrough 32D so as to diffract reflected light in the far-field in thedirection of the receiving optics 32A through 32D enables enhancedflexibility in the configuration and the arrangement of the receivingoptics 32A through 32D.

The present application is based on the Japanese Priority ApplicationNo. 2003-339564 filed on Sep. 30, 2003, the entire contents of which arehereby incorporated by reference.

1. A medium for optical recording, from which recorded information isreproduced by a laser beam, comprising: a disk board having a recordingsurface; and a plurality of pits in the recording surface, wherein eachof said pits is comprised in a corresponding one of a plurality of cellseach cell having equal size and varying pit-occupancy rates dependent onthe recorded information, said pit-occupancy rate being the ratio ofarea of said pit to area of said cell corresponding to said pit, whereindepths H of said pits, a wavelength λ of the laser beam, and arefractive index n of said board are related as: λ/6n<H<λ/4n, signalshaving a plurality of levels of N, N being dependent on thepit-occupancy rate, are generated, and said pits comprising (N−1)different pit diameters, the (N−1) pit diameters being set so as toalmost equally divide into N parts the difference between amount oflight reflected from the cells in a case of pits with pit diametershaving maximum values and amount of light reflected from the cells in acase of no pits existing.
 2. The medium for optical recording as claimedin claim 1, wherein modulation is 60% or above, said modulation being aratio of a signal level corresponding to the maximum of said different(N−1) pit diameters to a signal level corresponding to the minimum ofsaid different (N−1) pit diameters.